Integrated circuits incorporate complex electrical components formed in semiconductor material into a single device. Generally, an integrated circuit comprises a substrate upon which a variety of circuit components are formed wherein each of the circuit components are electrically isolated from each other. Integrated circuits are made of semiconductor material. Semiconductor material is material that has a resistance that lies between that of a conductor and an insulator. Semiconductor material is used to make electrical devices that exploit its resistive properties. A common type of semiconductor structure is the metal-oxide semiconductor (MOS).
Semiconductor material is typically doped to be either a N type or a P type. N type semiconductor material is doped with a donor type impurity that generally conducts current via electrons. P type semiconductor material is doped with an acceptor-type impurity that conducts current mainly via hole migration. A N type or P type having a high impurity or high dopant concentration or density is denoted by a “+” sign. A N type or P type having a low impurity or low dopant concentration or density is denoted by a “−” sign.
A capacitor can be formed using a MOS structure. A capacitor is a device that holds an electrical charge. A capacitor comprises a dielectric positioned between a top plate and a bottom plate. Typically, a capacitor of an integrated circuit is formed using the semiconductor material of a substrate as the bottom plate. An integrated circuit may comprise a plurality of capacitors created from a single substrate to form a circuit. Generally, MOS capacitors consume a relatively significant amount of area in many analog circuits.
A typical MOS capacitor formed in an integrated circuit comprises a heavily doped semiconductor surface as a bottom plate, a silicon dioxide (oxide) as a dielectric layer and a metal interconnect or poly layer as a top plate. The capacitance of this type of capacitor is given by the equation C=∈A/T, where C=capacitance, ∈=the dielectric constant of material that makes up the dielectric, A=the area of the capacitor plate and T=thickness of the dielectric. One method of reducing the capacitor area in an integrated circuit is by substituting silicon nitride (nitride) for the oxide as the dielectric. The dielectric constant of nitride is approximately 7/3.9 that of oxide. In addition, the two films, nitride and oxide, have about the same voltage blocking strength (rupture electric fields) so a layer of the same thickness can be used. Accordingly, the substitution of a layer nitride in place of a layer of oxide of the same thickness will reduce the capacitor area by about 44%.
Typically, the nitride is deposited over the entire substrate surface by low pressure chemical vapor deposition (LPCVD) or plasma enhance chemical vapor deposition (PECVD). The nitride is then patterned to form contact apertures or contact openings. This process usually, involves an extra pattering step when nitride overlays oxide that is not required when the oxide layer is used. Extra pattering steps add cost to the manufacture of integrated circuits. It is desired in the art to form an integrated circuit with reduced process steps.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for forming an integrated circuit having a MOS capacitor with reduced process steps.